A 16-year-old US boy with a ''blank expression'' stabbed and slashed 21 students and a security guard in the crowded halls of his suburban Pittsburgh high school before an assistant principal tackled him.
At least five students were critically wounded, including a boy who was on a ventilator after a knife pierced his liver, missing his heart and aorta by only millimeters, doctors said.
The rampage - which came after years in which US schools have geared much of their emergency planning toward mass shootings, not stabbings - set off a screaming stampede, left blood on the floor and walls, and brought teachers rushing to help the victims.
Police shed little light on the motive.
The suspect, Alex Hribal, was taken into custody and treated for a minor hand wound, then was brought into court in shackles and a hospital gown and charged with four counts of attempted homicide and 21 counts of aggravated assault. Authorities said he would be prosecuted as an adult.
The attack unfolded just minutes before the start of classes at 1200-student Franklin Regional High School, in an upper-middle-class area 24 kilometres east of Pittsburgh.
It was over in about five minutes, during which the boy ran wildly down about a hallway, slashing other students with kitchen knives about up to 25 centimeters long, police said.
Witnesses said the attacker at first tackled a freshman and stabbed him in the stomach, then got up and ran wildly down the hall, slashing other students.
Nate Moore, 15, said he saw the first attack and was going to try to break it up when the boy got up and slashed his face, requiring 11 stitches.
''It was really fast. It felt like he hit me with a wet rag because I felt the blood splash on my face. It spurted up on my forehead,'' he said.
The attacker ''had the same expression on his face that he has every day, which was the freakiest part,'' Moore said. ''He wasn't saying anything. He didn't have any anger on his face. It was just a blank expression.''
Assistant Principal Sam King finally tackled the boy and disarmed him, and a police officer who is regularly assigned to the school handcuffed him, police said.
Doctors said they expected all the victims to survive, despite large and deep puncture wounds to the abdomen in some cases.
King's son told The Associated Press that his father was treated at a hospital, though authorities have said he did not suffer any knife wounds.
''He says he's OK. He's a tough cookie and sometimes hides things, but I believe he's OK,'' Zack King said. He added: ''I'm proud of him.''
As for what set off the attack, Murrysville Police Chief Thomas Seefeld said investigators were looking into reports of a threatening phone call between the suspect and another student the night before. Seefeld didn't specify whether the suspect received or made the call.
Schools in the US have concentrated their emergency preparations on dealing with shooting rampages.
Nevertheless, there have been at least two major stabbing attacks at US schools over the past year, the first at a community college in Texas last April that wounded at least 14 people, and another, also in Texas, that killed a 17-year-old student and injured three others at a high school in September.
Mia Meixner, 16, said the initial assault Wednesday touched off a ''stampede of kids'' yelling, ''Run! Get out of here! Someone has a knife!''
Meixner and Moore called the attacker a shy and quiet boy who largely kept to himself, but they said he was not an outcast and they saw no indication before the attack that he might be violent.
''He was never mean to anyone, and I never saw people be mean to him,'' Meixner said. ''I never saw him with a particular group of friends.''
During the attack, the boy had a ''blank look,'' she said. ''He was just kind of looking like he always does, not smiling, not scowling or frowning.''
Michael Float, 18, said he had just gotten to school when he saw ''blood all over the floor'' and smeared on the wall near the main entrance. Then he saw a wounded student.
''He had his shirt pulled up and he was screaming, 'Help! Help!''' Float said. ''He had a stab wound right at the top right of his stomach, blood pouring down.''
Float said he saw a teacher applying pressure to the wound of another student.
Someone, possibly a student, pulled a fire alarm after seeing some of the stabbings, the police chief said. Although that created chaos, Seefeld said, it emptied out the school more quickly, and ''that was a good thing that that was done.''
Also, a girl with ''an amazing amount of composure'' applied pressure to a schoolmate's wounds and probably kept the victim from bleeding to death, said Dr. Mark Rubino at Forbes Regional Medical Center.
Gov. Tom Corbett commended the students in a visit to the stricken town, as well as cafeteria workers, teachers and teacher's aides who put themselves at risk to help during the attack.